This invention relates to the preparation of esters of fatty acids with hydroxyalkyl sulfonates, some of which are also known as isethionates. These compounds are well known as valuable synthetic lime soap dispersants and detergents, wetting agents and materials which are particularly useful in the manufacture of toilet soaps.
A variety of ways of making these compounds has been described in the art. U.S. Pat. No. 3,420,857 to Holland et al. and U.S. Pat. No. 3,420,858 to McCrimlisk disclose methods for the formation of fatty esters of hydroxysulfonates to obtain products which have reduced amounts of esters of higher molecular weight fatty acids and unreacted lower molecular weight fatty acids. The methods comprise continuously supplying to the reaction vessel, fatty acid reactants of a composition corresponding to fatty acids volatilized during the course of the reaction (in order to reduce the proportion of esters of relatively higher molecular weight fatty acids) and utilizing an improved stripping process to reduce the lower molecular weight fatty acid content. The method includes heating a mixture of an hydroxyalkylsulfonate and fatty acids to a temperature between about 390 degrees F. and 500 degrees F. (about 199-260 degrees C.). The examples are run at temperatures of at least 450 degrees F. (about 232 degrees C.). These patents note that temperatures below 450 degrees F. significantly reduce reaction rates. These patents also list a number of reaction promoters for the direct esterification reaction, including salts of strong acids and weak bases, zinc oxide and magnesium oxide, and acids and acid formers.
U.S. Pat. No. 3,429,136 to Holt et al. discloses a method for making esters of hydroxysulfonates in which the hot hydroxy-sulfonate esters are cooled from temperatures on the order of 350 degrees F. to 500 degrees F. (about 177-260 degrees C.), which are encountered in the preparation of such compounds, to a temperature below about 330 degrees F. (about 165.6 degrees C.). At this point the reaction is quenched by injecting cold water. The patent states that this quenching method is carried out without detectable amounts of hydrolysis.
U.S. Pat. No. 3,745,181 to Wrigley et al. discloses the preparation of 2-sulfoethyl esters of a number of fatty acids by acylating the sodium isethionate with the corresponding isopropenyl fatty ester by a transesterification reaction. The patent states that high purity products may be obtained using reaction times of 10-90 minutes and temperatures from 125-200 degrees C. Examples run at less than 200 degrees C., however, seem to result in decreased yields.
U.S. Pat. No. 4,405,526 to Lamberti et al. discloses a method for producing directly esterified fatty acylisethionates having a yellowness index less than about 6.0. The process consists essentially of reacting a fatty acid with an alkali metal isethionate in the presence of a catalyst comprising a mixture of zinc oxide (ZnO) and an organic sulfonic acid wherein the molar ratio of ZnO to organic sulfonic acid is about 1:1.7 or less and heating the reaction at about 200 degrees C. to about 225 degrees C. until the desired product is formed.
U.S. Pat. No. 4,515,721 to Login et al. discloses a process for the production of fatty acid esters of hydroxyalkyl sulfonate salts wherein the method comprises a) heating an excess of the fatty acids with the sulfonate until the water of condensation is removed; b) quenching the crude ester by immersion in an excess of cooled liquid in which the ester product is insoluble but in which unreacted, excess fatty acids are soluble; and c) filtering the slurry to separate the relatively pure ester. Isopropanol is taught as the preferred quenching liquid, but fatty alcohols (such as stearyl alcohol), fatty alcohol ethoxylates, polyethyleneglycols, fatty triglycerides (such as tallow or hydrogenated tallow), fatty esters and paraffins may also be used as the quenching liquid. The patent notes that the presence of a certain amount of such quenching liquids is acceptable and may actually facilitate detergent formulations. The method of this patent recites a temperature range of 200-250 degrees C., but all of the examples appear to be run at 250 degrees C.
U.S. Pat. No. 4,536,338 to Urban et al. discloses a method for preparing fatty acid isethionate soaps through direct esterification wherein the catalyst is quenched by an alkaline compound at the end of the esterification to inhibit transesterification between isethionate and later added stearic acids. The method comprises a) heating a mixture of C.sub.6 -C.sub.19 monocarboxylic acids with an hydroxysulfonate in the presence of a catalyst such as acidified zinc oxide, strong acids or soluble zinc salts; b) removing the liberated water; c) quenching the catalyst with an alkaline compound; and d) adding a higher molecular weight C.sub.15 -C.sub.24 fatty acid to the reaction mixture. The patent recites a reaction temperature of between 200 degrees C. and 260 degrees C. with 233 degrees C. being standard. The patent also mentions that increasing levels of zinc oxide to achieve faster rates of reaction gives a gritty feel to toilet bars made with the material.
German patent applications numbers 34 42 579 and 36 16 843 disclose a process for the esterification of carboxylic acids (RCOOH) with salts of hydroxyalkanesulfonic acids, wherein the R group of the acid is a saturated and/or unsaturated hydrocarbon of 7 to 31 carbons and the esterification takes place in the presence of a consistency regulator (such as paraffin) with a salt of the formula HO--(CH.sub.2).sub.n --SO.sub.3 X, where n is a number from 2-4 and X is an alkali metal or ammonium cation (NH.sub.4.sup.+). The 34 42 579 application states that the esterification is preferably carried out in a vacuum at temperatures of about 220-245 degrees C., particularly 225-235 degrees C.
The synthesis of one type of compound of this class, fatty acid esters of sodium acylisethionate, may be generally represented as follows: ##STR1##
The sodium isethionate (also called sodium hydroxyethane sulfonate) may be made by reacting ethylene oxide and sodium bisulfite.
While there have been a number of attempts at developing improved processes for making fatty acid esters of hydroxyalkyl sulfonates, there still remains a need for having a process that uses a direct esterification of salts of an hydroxyalkanesulfonic acid but that can be practiced at reduced temperatures, such as below 200 degrees C., without substantial reduction in yields or reaction rates. There also remains a need for producing fatty acid esters of hydroxysulfonates which have reduced problems with odor and color and which do not produce bars having a gritty feel when used to make toilet bars (or produce bars with reduced gritty feel).
Thus, it is an object of this invention to provide a process for the manufacture of fatty acid esters of hydroxyalkane sulfonates, which process may conveniently be run at temperatures not exceeding 200 degrees C. without substantial loss in yield or reduction in reaction rates. It is also an object of this invention to provide such a process with appreciable savings in energy costs compared to processes run at higher temperatures. It is another object of this invention to provide a process which produces fatty acid esters of hydroxyalkanesulfonates which have reduced odor and color problems, especially when the material is blended to make toilet bars. It is a further object of this invention to provide a process which produces fatty acid esters of hydroxyalkanesulfonates which have desirable lathering properties and reduced gritty feel when used to make toilet bars. These and other objects of the invention will be apparent from the following description.